1. Field of the Invention
The present invention relates to a thermal transfer sheet exhibiting excellent stability during preservation.
2. Prior Art
In recent years, a dye-sublimation thermal transfer recording method using a thermal transfer sheet having a thermal transfer dye layer, which contains a sublimation dye and which is formed on a base sheet thereof, has been widely used. A printing operation with the dye-sublimation thermal transfer recording method is performed in such a manner that the thermal transfer dye layer is heated in considerably short time to cause color dots in a multiplicity of colors and composed of the sublimation dye to be transferred to transfer paper. Thanks to the dots in the multiplicity of colors, a full color image of an original document can be reproduced. To realize a satisfactorily wide color reproducible range, indoaniline dyes have widely been employed in place of azo dyes, anthraquinone dyes and methine dyes.
A thermal head is brought into contact with the base sheet of the thermal transfer sheet so that heat required to perform the printing operation is applied. To prevent fusion of the thermal transfer sheet to the thermal head and to enable the thermal transfer sheet to move smoothly, a heat resisting and lubricating layer is formed on the reverse side of the base sheet (that is, a surface of the base sheet opposite to the thermal transfer dye layer).
The color density realized when the dye in the thermal transfer dye layer is transferred to the photographic paper is in proportion to the heating value which is applied from the thermal head. Therefore, the surface temperature of the thermal head must be changed in a range in units of some hundreds of degrees to realize satisfactory gradations in densities of each color. Therefore, the thermal transfer sheet and the thermal head move relatively while being in contact with each other. However, change in the surface temperature of the thermal head results in occurrence of change in the coefficient of friction between the thermal head and the heat resisting and lubricating layer of the thermal transfer. Thus, there arises the following problems.
If the coefficient of friction between the thermal head and the heat resisting and lubricating layer is changed, the thermal transfer sheet cannot easily be moved at predetermined speed. As a result, a sharp image cannot be formed. If the coefficient of friction is too large, the moving speed of the thermal transfer sheet is temporarily reduced. Thus, linear inconsistencies called "sticking" takes place which excessively raises the density of the formed image.
To prevent "sticking", the coefficient of friction at high temperatures must be reduced. As a lubricant for reducing the coefficient of friction at high temperatures, an attempt to use a phosphoric ester has been made.
If the indoaniline dye is employed as a sublimation dye in the thermal transfer dye layer in cyan, there arises a problem. That is, if a thermal transfer sheet comprising a heat resisting and lubricating layer, which contains a phosphoric ester lubricant having a high acidity, is rolled, the heat resisting and lubricating layer and the thermal transfer dye layer are brought into direct contact with each other. As a result, the indoaniline dye, which can easily be decomposed under an acidic condition, deteriorates. It leads to a fact that preservability of the thermal transfer sheet deteriorates and also the density of the printed image is lowered.